This invention relates to new and improved star (wye) connected, air gap, polyphase armatures for polyphase alternating current electric generators and motors and to the method of making the same.
More particularly, the invention relates to polyphase air gap type armatures as opposed to slot embedded armatures of the type having electrically grounded iron teeth between the coil conductors of the armature. In air gap type armatures, a graded-voltage armature can be formed by connecting adjacent winding turns of each phase in series electrical relationship in a manner such that potential increases nearly uniformly from coil to coil along the periphery of the active phase belt regions of each phase of the polyphase armature and no large voltage differences exist between physically adjacent coils within the phase belt. The invention makes it possible to build such polyphase air gap armatures wherein additionally no large voltage differences are produced between physically adjacent coil windings of the different phases at the phase boundaries, and is particularly useful in the construction and operation of cryogenic superconducting generator and motor armatures designed for transmission level voltages.
Many of the interconnection patterns known and commonly employed in fabricating conventional polyphase generator armatures produce a voltage pattern nearly suitable for a graded-voltage armature. However, in these known techniques and constructions, grounded iron teeth are interposed between the coil winding conductors to form what are called slot embedded armatures, and the presence of the grounded iron teeth requires each conductor to be insulated for bar-to-ground voltage. In contrast, in certain types of generator constructions, such as in a superconducting generator, it is possible to use only insulating materials between the adjacent coil conductors comprising the armature windings and a well-graded pattern of voltage differences can be established within each phase belt of the armature active annulus region.
If a graded-voltage armature is formed by connecting the adjacent winding turns of each phase group in series in a conventional lap or wave winding, the potential increases nearly uniformly from coil to coil along the periphery of each phase belt from one end of the phase belt to the other. If the armature is star (wye) connected, the regular progression of potential is interrupted at the boundaries between phase belts, at which point nearly full line-to-neutral voltage appears between the physically adjacent coil windings of the different phase belts. The space available for insulating against this voltage is so severely limited that this problem nearly rules out the use of conventional star (wye) connections for coil windings formed in this manner. The phase boundary voltage problem in graded-voltage wound armatures can be eliminated by delta connections, but this practice has other undesirable features as will be discussed hereinafter.
As a simple illustration of the above-discussed factors, reference is made to FIG. 1 of the drawings in conjunction with FIG. 2. FIG. 1 illustrates one phase group of a forty-eight coil, three-phase, double circuit, two pole, full pitch, two layer air gap armature wherein the winding pattern is a simple lap winding, conventionally connected in wye. FIG. 2 represents a cross-sectional view in the radial plane of the partial armature shown in FIG. 1. Each rectangle in FIG. 2 represents a coil side which can be considered to be the active region of the coil with the plurality of coil sides immediately adjacent each other forming the phase belt for the respective phase winding shown. The numbers identified with each coil side indicate the order of interconnection of the respective coil sides in series circuit relationship with the numbers having a superscript representing the prime circuit and the plain numbers the plain circuit. In considering the drawings, it should be kept in mind that a complete coil will be comprised of two coil sides, one on the radially-inner layer (solid line) and the other on the radially-outer layer (dotted line). From a consideration of FIGS. 1 and 2 it will be seen that the voltage between any two adjacent coil sides is nearly proportional to the difference between their numbers, taken without regard for the presence or absence of a superscript, and that the voltage difference between circumferentially adjacent coil in the same phase windings is seen to be 2/16th of the phase voltage.
The winding method and construction illustrated in FIGS. 1 and 2 possess a number of advantages. For example, all the coils are geometrically identical and have nearly identical electrical duties, so that all the coils may be made by essentially the same method. The pitch factor and the breadth factor are both satisfactorily high, and the factors limiting packing density of the insulated coils are no more restrictive than in a low voltage air gap armature.
The main shortcoming of the winding construction and method shown in FIGS. 1 and 2 stems from the fact that the coils at the circumferential extremities of each phase belt differ in potential by essentially the full phase voltage. Thus, if one terminal of the winding is made the neutral of a star connection (wye for three phase), the other circumferential edge of the winding is at line voltage. In polyphase armatures composed of a number of identical phase windings, the high voltage winding edge is of necessity physically adjacent the neutral winding edge of the next phase. The provision of adequate insulation at such phase boundaries is one of the most difficult problems in design of a graded-voltage armature.
In order to overcome the problems described in the preceding paragraph, polygon (delta in the three phase case) connected armatures have been proposed. However, polygon-connected armatures have other disadvantages compared to star (wye) connections, and as a result the vast majority of polyphase AC machines are star-connected. Some of the disadvantages of the polygon-connected armatures place constraints on the machine design. For example, third harmonic in the field winding flux will induce circulating currents in a delta-connected armature whereas in a star-connected armature no such comparable circulating currents can be induced. Other inherent disadvantages arise because of the restrictions placed on the design of the polyphase system to which such a machine is connected. These disadvantages then must be compared to the problem of insulating a star-connected generator to overcome the above-discussed high voltage differences across the phase boundaries.